Microstrip directional couplers are used for various microwave and radio frequency (RF) applications, including measuring signal power in a given system. Microstrip couplers are generally comprised of coupled transmission lines, including a main power line and a coupled line, wherein energy passing through the main transmission line is coupled to the coupled transmission line. The transmission lines are deposited onto the top of a substrate of electrically insulating material, with a conductive ground layer underneath the substrate. The microstrip coupler has forward propagating waves traveling from a source (such as a power amplifier, for example) to a load (such as an antenna, for example) and reverse propagating waves traveling in the load to source direction.
Waves propagating through microstrip lines have even and odd mode components. One measure of the quality of a microstrip coupler is the directivity of the coupler. The directivity of the coupler is the ability of the coupler to discern between the forward and reflected reverse waves in the transmission system for loads presented to the source. High directivity results from the even and odd mode waves propagating at identical or closely matched phase alignment, such that the waves arrive at the output terminals in phase. The effects of high directivity lead to a higher accuracy in measuring the voltage standing wave ratio (VSWR), which represents how well a source is matched to the load. The VSWR can range from 1:1 for a perfectly matched source and load (resulting in maximum power transfer from source to load) to infinity:1 for a perfect open or short circuit. The VSWR assists in determining when a load, such as an antenna, is degrading or out of specification.
In a conventional microstrip coupler, however, the odd mode phase velocity is faster than the even mode phase velocity such that the phases are out of alignment, thereby resulting in lower directivity. Poor directivity inhibits the accurate measurement of VSWR, thus making it difficult to distinguish different VSWRs to determine when a source and load are unmatched. Therefore, it is desirable to equalize the phase fronts of the even and odd modes, thus producing higher directivity and more accurate VSWR determination, which indicates how well the amplifier is matched to the load.
Several techniques have been previously developed to attempt to equalize the phases of the even and odd modes. One such technique has been to modify the shape of both the main line and the coupling line. Incorporating periodic structures into the main line, however, can cause the main line to deviate from its standard impedance. Previously attempted techniques have also required extensive redesign and modification of existing conventional microstrip couplers, resulting in greater processing or manufacturing variations that may further impact the quality of the coupler.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein.